How to get a List from a HashMap>

Question

I want to extract a List from a Map> (E is a random Class) using stream().

I

Answer 1

You can use Collection.stream with flatMap as:

Map> map = new HashMap<>(); // program to interface
List list = map.values()
                  .stream()
                  .flatMap(Collection::stream)
                  .collect(Collectors.toList());

or use a non-stream version as:

List list = new ArrayList<>();
map.values().forEach(list::addAll)